This invention relates to a holder for stabilizing the head of a subject who is prepared for undergoing a cranial X-ray examination. The holder is especially useful for computed axial tomography cranial examinations and will be described primarily in relation to that procedure.
As is well known, computed tomography apparatus is used for obtaining X-ray attenuation data from a thin body layer for the purpose of enabling reconstruction of an X-ray image that allows the layer to be viewed in an axial perspective. Computed tomography apparatus comprises an X-ray tube located on one side of a human body undergoing an X-ray scan and a multiple-element X-ray detector located on the other side of the body. The X-ray tube and detector are mounted on a gantry and are driven rotationally about a horizontal longitudinal axis so the tube and detector orbit the X-ray examination subject jointly. The X-ray beam emitted from the focal spot of the X-ray tube is collimated into a thin diverging or fan-shaped beam whose thickness corresponds with the thickness of the layer in the body being scanned. A common mounting for the X-ray tube and multiple-element detector is part of the gantry which permits the mounting and, hence, the rotational plane of the tube and detector to be tilted about a laterally extending horizontal axis to which the longitudinal axis is perpendicular. This permits making a scan and obtaining X-ray attenuation data for an image of a transfer slice or layer of the body which is at an angle relative to the vertical and to the longitudinal axis. It is necessary for the gantry to be constructed so the intersection point of the longitudinal and transverse axes do not shift in any direction when the rotational plane is tilted. The intersection point is called the isocenter. The longitudinal axis projects through the isocenter. A computed tomography gantry is described in U.S. Pat. No. 4,112,303 which is owned by the assignee of this application.
Typically, to make a cranial axial tomographic examination, the subject is supported in a supine position on an elongated X-ray transmissive cradle. The cradle is translatable so it overhangs the base on which it is mounted in cantilever fashion. A head holder or restraint is mounted near the end of the cradle for the purpose of stabilizing and maintaining the subject's head in a fixed position during the relatively long time which is required for scanning several adjacent cranial cross-sections or layers. It is important for the head of the subject to not move during the scan of a layer and that the head be in the identical position for each one of the successive scans. By way of example, it is usually desired that both optic nerves appear simultaneously in one of the axial perspective layers. The geometrical relationship of the subject's head to the plane of the fan-shaped X-ray beam is usually determined before the final tomographic scan is made. In other words, the subject is pre-positioned so that when the scan is made there is reasonable certainty that both optic nerves will be in view. A change in the subject's position of as little as 2 mm can then defeat the objective of imaging both optic nerves fully in the same axial view.
Some cranial X-ray studies may require the head of the subject to be in exactly the same position at the end of as much as forty minutes as at the beginning. Typically, the cranial study procedure involves making a computed projection radiograph which can be viewed to determine where and at what angle the fan-shaped scanning beam should be directed relative to the longitudinal axis of the subject's head. Minutes are often required in the decision-making process. As many as twenty layers may be scanned subsequently for obtaining the axial views. Although the cycle time per scan may be well under one minute, additional time may be consumed by waiting for an X-ray opaque medium which has been injected in the blood vessels to arrive at the proper place in the brain or cranium for getting the desired diagnostic information. Head holders that were available before the holder to be described later was invented have been found to be incapable of keeping the head of the subject in a precisely fixed position over a long period of time and are incapable of restoring the head to the predetermined desired position if there has been some movement over the long period.